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An L^1 ergodic theorem with values in a nonpositively curved space via a canonical barycenter map

By Andrés Navas


We extend a recent result of Tim Austin (see arXiv:0905.0515) to the L^1 setting, thus providing a general version of the Birkhoff ergodic theorem for functions taking values in nonpositively curved spaces. In this setting, the notion of a Birkhoff sum is replaced by that of a barycenter along the orbit. The construction of an appropriate barycenter map is the core of this note. In particular, we solve a problem raised by K.-T. Sturm showing that local compactness for the underlying space is superfluous for the construction (this extends a result of A. Es-Sahib and H. Heinich). As a byproduct of our construction, we prove a fixed point theorem for actions by isometries on a Buseman space.Comment: Final version: to appear in Erg. Theory and Dyn. System

Topics: Mathematics - Dynamical Systems, Mathematics - Group Theory, Mathematics - Probability
Year: 2011
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